Measurement of gain error in a base station receiver, and improvement in field strength measurement

ABSTRACT

For measuring gain error in a receiver of a base station of a cellular system and for improving field strength measurement, a signal received by the base station is measured before and after a receiver unit. The power level is corrected after the receiver unit to correspond to the power level of the signal determined before the receiver unit. For this purpose, before the receiver unit an accurate detector of the power level of the signal is provided, the power level of the signal detected by the detector being compared to the power level measured after the receiver.

BACKGROUND OF THE INVENTION

The present invention relates to a method for measuring gain error in areceiver of a base station of a cellular system and for improving fieldstrength measurement, this method comprising the steps of converting thefrequency of a signal generated in a transmitter unit to be adapted to areceiver unit and passing the signal to the receiver unit, andgenerating a control signal from the signal for indicating fieldstrength.

The invention further relates to equipment for measuring gain error in areceiver of a base station of a cellular system and for improving fieldstrength measurement, this equipment comprising a transmitter unit,which comprises a transmitter and a power regulator of the transmitter;a receiver, which comprises a receiver unit, a distribution amplifierand a band-pass filter; a numerical signal processing unit; and acontrol unit, which controls the transmitter unit and the receiver.

Gain variation refers in this context to the deviation of the actualgain of the receiver unit from the nominal value of the gain. This gainvariation can be as wide as about ±10 dB at a base station, due to thewide frequency range, temperature variations and inaccuracies incomponent values. As regards the operation of a cellular system,variations this wide are nevertheless harmful because, for example, GSMrequires that the base station should continuously measure the strengthof received signals during normal traffic. This is done by means ofso-called RSSI calculations (Received Signal Strength Indication),whereby the signal level of a received signal is calculated on the basisof a signal obtained from the output of the base-station receiver. It isapparent that the calculated RSSI value deviates significantly from theactual one if the gain variation of the receiver is as wide as ±10 dB,and the nominal value of the gain is used in the calculations.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method and equipmentfor measuring the gain error in a base-station receiver and for makingthe RSSI value more accurate.

This method is characterized in that the power level of the signalobtained from the transmitter unit is determined before the receiver;that the power level of the signal is determined after the receiver;that the power levels of the signal determined before and after thereceiver are compared to one another; and that the control signal valueindicating field strength is corrected to correspond to the power levelof the signal as determined before the receiver.

The equipment of the invention is further characterized in that thereceiver comprises a detector element for detecting the power level ofthe signal coming from the transmitter unit; that after the receiverthere is a detector element for determining the power level of thesignal; that the control unit comprises a comparator element forcomparing the power levels of the signal before and after the receiver;and that the control unit comprises a correction element for correctingthe control signal indicating field strength so that it will correspondto the power level of the signal as determined before the receiver.

An essential idea of the invention is that the transmitter signal isapplied directly to an available channel in the receiver and that thestrength of the received test signal is measured after the receiver.Next, the power level of the test signal transmitted from thetransmitter and the power level of the output signal of the receiver arecompared to one another, this comparison yielding the gain between theinput and output of the receiver, and thus it is possible to determinethe gain error in the receiver from this gain. Another essential idea isto arrange an accurate threshold detector of the power level of thesignal as close as possible to the input of the receiver, whereby it ispossible to adjust the level of the transmitter test signal veryaccurately as desired by means of the level information produced by thethreshold detector. Similarly, it is possible to compare the power levelinformation produced by the threshold detector of the receiver to thepower level of the receiver output signal and to form a differencevariable from the difference of these signals, and to compensate for thesignal indicating field strength by this difference. Yet anotheressential idea is to connect the test signal of the transmitter unit tothe receiver at the earliest possible stage, whereby a highly accuratepicture of the total gain error in the receiver is formed.

An essential advantage of the invention is that the gain, and at thesame time the gain error, in the receiver can be monitored in use, withno need for separate measuring instruments in this arrangement. Afurther essential advantage is that in repairing the base station aftera failure, it is possible to determine the gain automatically and totake into account a possible deviation of the gain from the nominalvalue when calculating the field strength.

The invention is further described in the following with reference tothe attached drawings, in which:

FIG. 1 shows a control principle of the transceiver system of theinvention,

FIG. 2 shows another control principle of the transceiver system of theinvention, and

FIG. 3 shows a third control principle of the transceiver system of theinvention.

DETAILED DESCRIPTION

FIG. 1 is a schematic representation of a sub-unit 1 of a base-stationtransceiver system in a cellular system, this sub-unit comprising atransmitter unit 2, which is composed of a transmitter TX and a powerregulator TX Power Control of the transmitter TX. In the drawings, thetransmitter TX is indicated by reference numeral 3 and the powerregulator TX Power Control of the transmitter TX by reference numeral 4.The transmitter unit 2 is connected to an antenna 5 via a band-passfilter 6. The receiver unit is indicated by reference numeral 7. Inaddition to the receiver unit 7, the receiver comprises a band-passfilter 9 and a distribution amplifier 10. Immediately after the receiverunit 7 is connected a digital signal processor DSP, which is used fornumerical signal processing and which is indicated by reference numeral8. The receiver unit 7 is connected to the antenna 5 via the band-passfilter 9 and the distribution amplifier 10. The function of thedistribution amplifier 10 is to divide a signal coming from thedirection of the antenna 5 into two or more branches, to each of whichis connected the receiver unit 7 according to FIG. 1. The branching inFIG. 1 is indicated by lines 11a, 11b and 11c marked with arrows, theline 11c being connected to a directional coupler 12. To the directionalcoupler 12 is connected an accurate threshold detector 13, by which thepower level of an incoming signal can be determined accurately and withreference to predetermined thresholds. To the threshold detector 13 isconnected a receiver amplifier 14, the function of which in FIG. 1 is todetect different gains in the receiver unit 7 that are due for instanceto variation in component values, and also to detect the operation ofthe actual amplifier. The received signal is then passed to the digitalsignal processing unit 8, which converts the analog data to digital datain a block A/D, which is indicated by reference numeral 15 and which islater referred to as A/D conversion block. Numerical processing of thesignal converted to a digital form is carried out in an RSSI calculationunit 16, the power level of the incoming signal being recalculated alsoin a digital form. Similarly, the transmitter 3 comprises a directionalcoupler 17, by which a sample signal D of a lower power is taken fromthe signal going to the antenna 5, to be used as a test signal by amixing unit 18. The ratio between the power of the transmitter signalgoing to the antenna 5 and that of the sample signal D remains constant.To the mixing unit 18 is connected a local oscillator 19, the testsignal D coming from the transmitter 3 being mixed in a frequency mixer20 by a signal E produced by the local oscillator 19. Thus, thefrequency of the test signal D of the transmitter 3 is suitable to bepassed via the directional coupler 12 to the threshold detector 13 andthen to the receiver unit 7. The operation of the transmitter unit 2,the receiver unit 7 and the mixing unit 18 is controlled by atransceiver controller TRX Control, indicated by reference numeral 21 inFIG. 1. The transceiver controller 21 adjusts the power level of anoutgoing signal by means of the power regulator 4 of the transmitterunit. Since using the power regulator 4 of the transmitter 3 foradjusting the power level of the test signal nevertheless results in arelatively coarse accuracy, about 3 dB, a precision regulation block 22is connected to the mixing unit 18 for accurate power level regulationof the test signal, it being thus possible to adjust the power level ofthe test signal by said precision regulation block typically with anaccuracy of about 1/2 dB. Respectively, the precision regulation block22 is monitored and controlled by the transceiver controller 21. Thepower level of the test signal is measured by the accurate thresholddetector 13 and, by means of this data, the transceiver controller 21regulates either the power regulator 4 of the transmitter unit 2 or theprecision regulation block 22 in order to obtain a desired power levelfor the test signal. Rough power level adjustment is indicated by arrowA, fine power level adjustment by arrow B, and level data passed by thethreshold detector 13 to the transceiver controller 21 by arrow C.Similarly, a test signal going out from the transmitter is indicated byarrow D, a signal generated by the local oscillator 19 by arrow E, and asignal produced by the interaction of the signals D and E by arrow F.The digital data on the signal power level produced in the RSSIcalculation unit 16 is compared in the transceiver controller 21 to thepower level data C produced by the threshold detector 13. Thetransceiver controller 21 can correct the RSSI value on the basis of theobtained difference. The traffic between the RSSI calculation unit 16and the transceiver controller 21 is thus bidirectional and is indicatedin the figure by arrows G₁ and G₂, the arrow G₁ denoting an uncorrectedRSSI value and the arrow G₂ gain error data that is passed to the RSSIcalculation unit 16. From the RSSI calculation unit 16 is obtained asignal which is proportional to field strength and which does notinclude the gain errors of the amplifier 14 of the receiver unit 7. Theobtained data is indicated by arrow H, which denotes a corrected RSSIvalue.

FIG. 2 shows another sub-unit of the transceiver system of theinvention. The numbering in FIG. 2 corresponds to that in FIG. 1. FIG. 2shows a sub-unit 1 of the transceiver system, a transmitter unit 2, atransmitter 3, a power regulator 4 of the transmitter 3, an antenna 5, aband-pass filter 6, a receiver unit 7, a digital measuring unit 8, aband-pass filter 9, a distribution amplifier 10, lines 11a-11c, adirectional coupler 12, an accurate threshold detector 13, an amplifier14 of the receiver unit 7, an A/D conversion block 15, a directionalcoupler 17 of the transmitter 3, a mixing unit 18, a local oscillator19, a frequency mixer 20, a transceiver controller 21 and a precisionregulation block 22. Similarly, the functions and information indicatedby signals A, B, C, D, E, F, G₁, G₂ and H correspond to the descriptionof FIG. 1. In the case of FIG. 2, the accurate threshold detector 13 is,however, provided before the distribution amplifier 10, whereby thesignal power level data C produced by the threshold detector 13 iscompared to the digital power level data G₁ produced by the RSSIcalculation unit 16. This allows the gain error produced by thedistribution amplifier 10 to be taken into account and to be compensatedfor, whereby the signal H indicating field strength will provide veryaccurate data corresponding to the actual field strength. Further, it ispossible to carry out said level regulation so that the thresholddetector 13 is replaced by a detector operating in a narrow level range,whereby it is possible to determine the gain error by adjusting thepower level of the test signal D transmitted by the transmitter 3 in asituation where the level of the test signal D of the transmitter 3 isin the same range with the narrow-band level detector, andsimultaneously, to determine the digital RSSI value from the RSSIcalculation unit 16. The level data G₁ obtained from the RSSIcalculation unit 16 is then compared to the threshold values of thenarrow-band level detector, and the RSSI value is corrected by using thedata so as to be appropriate.

FIG. 3 shows a third regulating unit that measures and corrects the gainerror in a receiver of a cellular system. Numbering and symbols in FIG.3 fully correspond to those in FIGS. 1 and 2. In FIG. 3, the thresholddetector 13 is placed after the mixing unit 18, whereby the power levelof the test signal D transmitted from the transmitter 3 can be measuredvery accurately after the frequency conversion of the test signal D. Thelevel data C produced by the threshold detector 13 is compared to thepower level data which is produced in the RSSI calculation unit 16 andwhich can be corrected to correspond to the power level data on the testsignal F. It it thus possible by said arrangement to compensateaccurately also for errors caused by the directional coupler 12 in thegain of the system.

The drawings and the description relating to them are intended only toillustrate the idea of the invention, and the invention according to theapplication can vary within the scope of the claims.

We claim:
 1. A method for measuring gain error in a receiver of areceiver unit of a base station of a cellular system and for improvingfield strength measurement, comprising the steps of:generating a signalby a transmitter of a transmitter unit of the base station, andobtaining a sample signal proportional in power thereto; adapting saidsample signal to said receiver unit and passing the adapted samplesignal to said receiver unit, said adapting including converting thefrequency of said sample signal to cause said adapted sample to beappropriate for reception by said receiver unit; determining the powerlevel of said adapted sample signal before and after said receiver toprovide before and after power levels; comparing said before and afterpower levels with one another; generating a control signal indicative offield strength from said adapted sample signal as received by saidreceiver; correcting the value of said control signal based on saidbefore power level; said base station including a distribution amplifierinterposed between an antenna and said receiver, and said passingcomprising applying said adapted sample signal before said distributionamplifier, and said determining being accomplished from before saiddistribution amplifier and after said receiver unit.
 2. The method ofclaim 1, wherein:said adapting includes mixing said sample signal withanother signal, in a mixing unit.
 3. The method claim 1, wherein:saidadapting includes converting the frequency of said sample signal tocause said adapted sample signal to be appropriate for reception by saidreceiver unit; said passing comprises applying said adapted samplesignal to said receiver immediately before said receiver unit; and saiddetermining is accomplished from immediately before and immediatelyafter said receiver unit.
 4. The method of claim 1, furthercomprising:said adapting includes comparing the sample signal withpredetermined threshold values of power level, and adjusting the powerlevel of said adapted sample signal so as to be between saidpredetermined threshold values.
 5. The method of claim 1, furthercomprising:measuring the power level of said sample signal.
 6. Themethod of claim 5 further comprising:converting said adapted samplesignal from analog to digital form after said receiver; and performingsaid measuring on said digital from of said adapted sample signal. 7.Equipment for measuring gain error in a receiver of a base station of acellular system and for improving field strength measurement,comprising:a transmitter unit comprising a transmitter arranged forgenerating a signal regulator for said transmitter; a receiver unitcomprising a receiver, a distribution amplifier serving said receiver,and a band-pass filter serving said distribution amplifier; a signalprocessing unit located between said transmitter unit and said receiverunit, and providing as output a signal proportional to field strength ofsaid signal generated by said transmitter; a control unit arranged forcontrolling said transmitter unit and said receiver, a subsystemarranged for supplying a sample signal proportion in strength to saidsignal generated by said transmitter, to said receiver; a first detectorarranged before said distribution amplifier, for detecting the powerlevel of said signal generated by said transmitter, before saidreceiver; a second detector, located after said receiver, and arrangedfor determining the power level of said signal generated by saidtransmitter, after said receiver; said control unit comprising acomparator element for comparing with one another said power levels asdetected by said first and second detectors; said control unitcomprising a correction element arranged for correcting said signalproportional to field strength, based on the power level detected bysaid first detector.
 8. The equipment of claim 7, further including:saidsubsystem includes a mixing unit interposed between said transmitterunit and said receiver and arranged for causing said sample signal tohave a frequency appropriate for reception by said receiver.
 9. Theequipment of claim 7, wherein:said first detector is arrangedimmediately before said receiver unit.
 10. The equipment of claim 7,wherein:said first detector is arranged for detecting whether the powerlevel of said signal generated by said transmitter is betweenpredetermined threshold values; said transmitter unit includes a powerregulator roughly adjusting the power level of said signal generated bysaid transmitter; and said subsystem includes a mixing unit interposedbetween said transmitter unit and said receiver and arranged for causingsaid sample signal to have a power level which is proportional to onebetween said predetermined thresholds values.
 11. The equipment of claim7, wherein:said first detector is a measuring device arranged foraccurately measuring the power level of said sample signal.
 12. Theequipment of claim 7, wherein:said signal processing unit includes ananalog to digital converter arranged to convert the sample signal, aftersaid receiver into digital form; and said second detector is arrangedfor determining the power level of said signal generated by saidtransmitter, by detecting the power level of said sample signal afterconversion of said sample signal by said analog to digital converter todigital form.